Süssmayr was a student of Wolfgang Amadeus Mozart. During Mozart's last days they discussed his Requiem and Süssmayr took on the task of completing the piece upon his death. Though criticised by some as not Mozartian, it is standard to play Süssmayr parts (for he composed the last fifth) in performances.

Mozart composed:
- Introïtus
- Kyrie
- Dies irae
- Tuba mirum
- Rex tremendae
- Recordare
- Confutatis
- Lacrymosa
- Domine Jesu
- Hostias
The other parts are composed/ finished by Süssmayr:
- Sanctus
- Benedictus
- Agnus Dei
(The Communio repeats the music of the Introitus and Kyrie.)
But much is unsure.
Despite the complexity of its origins – its composition during the final days of Mozart and completion after his death by F.X. Süssmayr (with some input from J.L. Eybler and, possibly, F.J. Freystädtler) – Mozart's Requiem is the most important example from the 18th century. The exact extent of Mozart's contribution is still debated; but such stylistic unevenness as may have resulted from additions by others has hardly lessened the impact of the work as a whole.